The invention is based on a circuit for limiting the beam current of a cathode ray tube in a television receiver.
In a television receiver, it is known and necessary to limit the beam current flowing in the cathode ray tube, in order to protect the cathode ray tube and the high-tension transformer. If the beam current is excessively high over a relatively long time period, the guaranteed life of the cathode ray tube is at risk, while, in the high-tension transformer, the high-tension rectifier diodes, which are potted together with the windings, can, in particular, be destroyed by an excessively high temperature.
In circuits for limiting the beam current, only one limiter function is in general active. In order to display briefly occurring white image points with adequate brightness, the limit value is set such that it is above the beam current at which the cathode ray tube may be loaded continuously, without having to accept damage. In normal operation with moving images, the long-term average is always below this maximum limit of the beam current, since there are also dark scenes in between, in which the limiting does not act. However, continuous operation with active beam current limiting is theoretically possible.
The, dimensioning of the beam current limiting circuit has to take account of an extreme load case. This occurs, for example, if the television receiver is operated at an ambient temperature of +40.degree. C. over a number of hours with a completely white image in extremely poor cooling conditions, for example in a bookcase. Even in this extreme load case, the limit values for the high-tension transformer must not be exceeded. The extreme load case admittedly occurs very rarely, or even never, with a unit in practice. The necessity to take account of this in the dimensioning of the beam current limiting constrains the freedom, however, for the maximum permissible beam current for normal moving images. In other words: if it were possible to be certain that the said extreme load case would not occur, a higher beam current could be permitted for normal moving images, and a brighter image could thus be achieved.